Many drugs have been developed to prevent blood clots from forming. Anti-coagulants, including aspirin, warfarin, and heparin, are widely used for the purpose. There are, however, very few therapies that can break down clots already formed. The process of dissolving a clot is called thrombolysis.
Thrombolytic agents work directly upon the fibrin strands within a clot. Examples include plasma activator agents, which increase plasma activator activity, and plasminogen activators, such as streptokinase, urokinase and tissue plasminogen. All these agents digest clots by increasing the amount of plasmin in the blood. Plasminogen is activated and converted into plasmin by enzymes called plasminogen activators, increasing the level of plasmin available to digest clots.
Streptokinase has been utilized in medicine since the 1960s. The least expensive plasminogen activator, streptokinase has caused negative side effects in some patients, such as immune responses. Urokinase is a naturally-occurring protein in humans, especially in the urine. There is therefore no negative immune response associated with its use. Urokinase therapy is usually administered in small doses in combination with other drugs, because it is difficult to purify and therefore rather expensive.
Tissue plasminogen activator (tPA) is an expensive drug for dissolving blood clots. It is unique because it activates only fibrin-bound plasminogen and thus targets clot sites. tPA in human blood is, however, produced in very small amounts by vascular endothelial cells. Recombinant DNA technology has therefore been utilized to make copies of the gene that encodes human tPA. These genes are then transfected into host cells for production of the corresponding proteins. Chinese hamster ovary cells are often utilized for protein production. Recombinant plasminogen must still be converted to plasmin to be effective. Because the overall process can be lengthy and require a large amount of tPA, which is costly, it is desirable to find alternative methods of dissolving blood clots.